The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2001-177535 filed on Jun. 12, 2001 the entire contents thereof is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a frame structure of a motorcycle in which a connecting tube for introducing air from an air cleaner into a carburetor is disposed in the vicinity of the rear end portion of the vehicle body frame.
2. Description of the Background Art
The motorcycle includes a vehicle body frame as a framework for supporting an engine and other members. A vehicle body frame is disclosed in Japanese Patent Laid-Open No. 84190/1995 entitled xe2x80x9cFRAME STRUCTURE IN VEHICLES SUCH AS A MOTORCYCLE.xe2x80x9d This frame structure will be described below referring to FIG. 3 of this publication which corresponds to FIG. 9 in the drawings.
FIG. 9 is a side view of the motorcycle including a frame structure of the related art, showing a state in which a vehicle body frame 150 of a motorcycle comprises a pair of left and right main pipes 152, 153 extending from the head pipe 151 to the rear of the vehicle body. A pair of left and right brackets 154, 155 are joined to the rear end portion 152a, 153a of the main pipes 152, 153 respectively. A pair of left and right down tubes 156, 157 extend from the lower end portion 154a, 155a of the bracket 154, 155 to the front of the vehicle body.
An engine 160 is mounted between the left and right main pipes 152, 153, and the left and right down tubes 156, 157. A carburetor 161 is connected to the engine 160 with a connecting tube 162 extending from the carburetor 161 along the rear end portion 152a of the left main pipe 152. An air cleaner 163 is connected to the connecting tube 162.
Accordingly, air from the air cleaner 163 is introduced into the carburetor 161 via the connecting tube 162. The introduced air may be supplied to the engine after fuel is mixed with the air.
An important requirement for maintaining optimum engine performance of the motorcycle is to supply air to the carburetor 161 in a suitable manner. In order to supply air to the carburetor 161 in a suitable manner, it is necessary for the air to flow from the air cleaner 163 smoothly in the connecting tube 162.
In order to ensure smooth flow of air in the connecting tube 162, it is necessary to form the flow path of the connecting tube 162 in a naturally curved shape, and in order to form it in a naturally curved shape, it is necessary to reserve an enough space for disposing the connecting tube 162.
Since the rear end portion 152a of the left main pipe 152 is exposed to the space where the connecting tube 162 is to be disposed, whether or not enough space can be reserved largely depends on the shape of the left main pipe 152.
The left and right main pipes 152, 153 constituting the vehicle body frame 150 of a motorcycle generally tends to use many extruded moldings, and thus the thickness of the cross section of the left main pipe 152 is constant from the front end portion 152b through the rear end portion 152a. 
In addition, the left and right main pipes 152, 153 have to be thick enough to ensure rigidity of the vehicle frame 150. Therefore, the thickness of the rear end portion 152a of the left main pipe 152 increases, and thus enough space for disposing the connecting tube 162 cannot be reserved.
Therefore, the flow path of the connecting tube 162 cannot be formed into a naturally curved shape, and thus smooth flow of air in the connecting tube 162 cannot be ensured, which makes it difficult to maintain the engine performance of the motorcycle in a suitable condition.
Accordingly, it is an object of the present invention to provide a frame structure for a motorcycle in which the connecting tube can be configured in a shape that ensures smooth flow of air.
In order to achieve the aforementioned object, the present invention is directed to a motorcycle wherein a pair of left and right mainframes extend from a head pipe to the rear of the vehicle body. A pivot plate is joined to the rear ends of the mainframes. A rear swing arm extends rearwardly from the pivot plate. An air cleaner is mounted above the rear swing arm with the air cleaner being connected to the carburetor via a connecting tube. The carburetor is connected to the engine located below the mainframe. The pivot plate is a member formed by casting or forging wherein the pivot plate extends to a position above the rear end portion of the mainframe so that a prescribed portion of the pivot plate faces towards the outer surface of the connecting tube. The prescribed portion is formed with a recess for accommodating the outer surface of the connecting tube and a rear shock absorber is provided for supporting the rear swing arm that is mounted on the upper end portion of the pivot plate extending to the position above the mainframe.
By employing a member formed by casting or forging as a pivot plate, a recess for accommodating the outer surface of the connecting tube may be formed at a prescribed portion on the pivot plate during the process of forming the pivot plate. In this manner, by forming a recess at the prescribed portion, a space for disposing the connecting tube may be increased.
By reserving enough space, freedom of determination of the shape of the connecting tube increases and thus the connecting tube may be formed in a shape that allows smooth flow of air therein.
The present invention is directed to including an end face of the rear end portion of the mainframe that is inclined in such a manner that the upper portion is positioned forward of the lower portion.
By inclining the end face of the rear end portion of the mainframe in such a manner that the upper portion is positioned forward of the lower portion, the end face of the rear end portion of the mainframe may be tipped forward to set the mainframe apart from the connecting tube.
By setting the mainframe apart from the connecting tube, the prescribed portion of the pivot frame may face towards the outer surface of the connecting tube. By forming a recess on the prescribed portion, the space in which the connecting tube is to be disposed may be increased.
Therefore, the freedom of determining the shape of the connecting tube may be increased.
In addition, by tipping forward the end face of the rear end portion of the mainframe, the joint between the mainframe and the pivot plate may be elongated. Therefore, the strength of the joint may be increased.
The present invention includes a rear end portion of the mainframe that is positioned in the vicinity of the rear end portion of the carburetor.
By positioning the rear end portion of the mainframe in the vicinity of the rear end portion of the carburetor, the rear end portion of the mainframe may be set apart from the connecting tube more positively, so that the prescribed portion of the pivot frame may face towards the outer surface of the connecting tube. By forming a recess on the prescribed portion, the space for disposing the connecting tube may be increased. Therefore, the freedom of determining the shape of the connecting tube is increased.
The present invention includes a rear end portion of the mainframe that is positioned forward of the rear end portion of the carburetor.
By positioning the rear end portion of the mainframe forward of the rear end portion of the carburetor, the rear end portion of the mainframe may be set apart from the connecting tube more positively. Therefore, the prescribed portion of the pivot frame may easily face towards the outer surface of the connecting tube.
Therefore, since the prescribed portion of the pivot frame may face towards the outer surface of the connecting tube relatively easily, the recess may be provided at the suitable position.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.